CNF Cathode Catalyst for Li-O2 Batteries

Accepted Manuscript Title: Enhanced Electrochemical Performance of Nanofibrous CoO/CNF Cathode Catalyst for Li-O2 Batteries Author: Bo-Wen Huang Lei Li Yi-Jun He Xiao-Zhen Liao Yu-Shi He Weiming Zhang Zi-Feng Ma PII: DOI: Reference:

S0013-4686(14)01115-3 http://dx.doi.org/doi:10.1016/j.electacta.2014.05.114 EA 22808

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

1-4-2014 25-5-2014 25-5-2014

Please cite this article as: B.-W. Huang, L. Li, Y.-J. He, X.-Z. Liao, Y.-S. He, W. Zhang, Z.-F. Ma, Enhanced Electrochemical Performance of Nanofibrous CoO/CNF Cathode Catalyst for Li-O2 Batteries, Electrochimica Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.05.114 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced Electrochemical Performance of Nanofibrous CoO/CNF Cathode Catalyst for Li-O2 Batteries

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Bo-Wen Huang1, Lei Li1, Yi-Jun He1, Xiao-Zhen Liao1,*, Yu-Shi He1, Weiming Zhang1,2 and Zi-Feng Ma1,2,*

Institute of Electrochemical and Energy Technology, Department of Chemical

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Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Sinopoly Battery Research Centre, Shanghai 200241, China

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* Corresponding author. E-mail: [email protected]; [email protected]. Abstract

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A high performance CoO/carbon nanofibers (CNF) composite catalyst was synthesized for Li-O2 batteries. For comparison, CoO/BP2000 and CoO/MWNTs

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were also prepared and investigated to study the influence of carbon supports on the electrochemical performance of the composite catalysts. Electrochemical tests showed

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that the Li-O2 battery with CoO/CNF demonstrated obviously enhanced

electrochemical performance than the batteries with CoO/BP2000 and CoO/MWNTs catalysts, which delivered a first discharge capacity of 3882.5 mAh gcat-1 and remained about 3302.8 mAh gcat-1 after 8 cycles in the voltage range from 2.0 to 4.2 V.

More importantly, the cycle stability of the Li-O2 battery with CoO/CNF could maintain over 50 cycles when cycled at a fixed capacity of 1000 mAh gcat-1. The unique porous nanofiberous structure of CoO/CNF greatly contributed to its high electrocatalytic performance. Key words: carbon nanofibers, CoO/CNF composite, Li-O2 battery, electrocatalytic 1

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performance 1. Introduction In recent years, rechargeable Li-O2 battery has attracted great interest for the

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development of energy storage systems and vehicle energy [1-4]. The Li-O2 battery

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has a theoretical energy density high up to 5200 Wh Kg-1 (including the mass of O2),

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close to the energy density of fossil energy sources [4-6]. It is expected that the environmentally friendly Li-O2 battery could be widely applied as a new energy

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resource to replace the diminishing fossil energy in the future. However, the challenge problems of the current Li-O2 battery system including low round-trip efficiency, poor

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cycling stability and rate capability have to be resolved before its practical application [6-8]. In a Li-O2 battery, Li+ ion reacts with O2 at cathode producing Li2O2 or Li2O

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during discharge. Reversibly, oxygen is evolved at cathode during charge [1, 9-10].

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Three possible reactions between Li+ and oxygen have been considered in the

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discharge process [9, 11-13]: Li+ + O2+ e-→ LiO2

(Erev = 3.0 V vs. Li/Li+)

(1)

2 Li+ + O2 + 2e-→ (Li2O2) solid

(Erev = 3.1 V vs. Li/Li+)

(2)

4 Li+ + O2 + 4e- → 2(Li2O) solid

(Erev = 2.91 V vs. Li/Li+)

(3)

It is obvious that the electrochemical reactions described above are typical “oxygen reduction reactions” (ORR) [3, 14, 15]. During the discharge process of the Li-O2

battery, O2 must continuously diffuse into the porous air electrode to take part in the electrocatalytic reactions. Accordingly, both the effective O2 diffusion and the sufficient “electrochemical reaction interface” formed by O2-catalyst-electrolyte 2

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solution are extremely crucial to the interface electrocatalytic reactions [16-18]. Therefore, it is very important to fabricate an excellent porous air electrode with a high “electrochemical reaction interface” area to improve the electrochemical

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performance of the Li-O2 battery.

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For this purpose, many efforts have been made to optimize the structure and

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composition of the air electrodes to improve the “electrochemical reaction interface”. The carbon or carbon supported composites with large surface area, high electric

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conductivity and open structures, are often used as outstanding catalysts and demonstrate good catalytic performance in the Li-O2 batteries, such as XC-72

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carbon-TiN [14], N-rich carbon-Co [19], Activated Carbon (AC) [20], Ketjen black-Co3O4 [21], woven carbon-(Co-Mn) [22], Carbon-MnOx [23], Diamond like

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carbon (DLC)-CoOx [24] and mesoporous carbon (CMK-3)-CoO [25]. On the other hand, some one-dimensional nanowire, nanofiber and nanotube materials with a

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network were also investigated as effective catalysts in the Li-O2 batteries. It was reported that the network structure of one-dimensional carbon materials could facilitate the forming of a random interconnected pore structure [18, 26-29], in which both the gas and electrolyte can be easy to diffuse or transfer [6-7, 30]. Recently, non-carbon based porous materials were also synthesized and showed enhanced catalytic activities, such as porous nano-polypyrrole (PPy) nanotubes[31] and porous La0.75Sr0.25MnO3 nanotube [32] for the ORR in the Li-O2 battery. In our previous work, we have successfully synthesized a serials of porous carbon nanofiber based materials via one-step electrospinning technique and 3

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subsequent controlled carbonization process [6]. In addition, it was demonstrated that the cobalt and its oxides CoOx (such as Co, CoO and Co3O4) could be used as effective catalysts for the oxygen electrodes [19, 21, 24-25, 30, 33-34]. In this work,

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we present the electrochemical performance of a CoO/carbon nanofibers composite as

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a new cathode catalyst for the Li-O2 battery. A schematic diagram for the fabrication

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of the CoO/CNF composite is depicted in Figure 1. The electrocatalytic performance of the prepared CoO/CNF cathode catalyst was compared with the CoO/MWNTs and

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CoO/BP2000 composite catalysts to investigate the influence of carbon supports.

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2. Experimental

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2.1. Preparation and Characterization

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The electrospun CoO/CNF nanofibers were prepared as follows: Firstly, 1.0 g

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Cobalt (II) acetate tetra-hydrate (Sigma-Aldrich) was added into 10 mL N, N-dimethylformamide (DMF) solvent (Sigma-Aldrich). After complete dissolution, 1.0 g Polyacrylonitrile (PAN, 15, 0000 g mol-1, Sigma-Aldrich) was gradually added

into the mixed solution with continuous magnetic stirring for 10 h to form a homogeneous viscous solution. Then, the prepared solution was loaded into a 2 mL plastic syringe and electrospun at a steady voltage potential of 15 KV with a flow rate

of 0.2 mL h-1 at room temperature and 35% RH. During the electrostatic spinning process, the distance between the spray nozzle and aluminum foil collector was maintained about 12 cm. The Co(OAc)2/PAN electrospun nanofibers were collected on the aluminum foil and dried at 80 ℃ for 12 h in a vacuum oven before the 4

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subsequent thermal treatment. In the process of controlled thermal treatment, the dried Co(OAc)2/PAN fibers were firstly pre-oxidized in air atmosphere at 230 ℃ for 10 h, and then the pre-oxidation product was carbonized at a high temperature of 900 ℃

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for 1h in high purity argon (99.99%) flow.

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For comparison, CoO/MWNTs and CoO/BP2000 composite were also prepared

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using the general synthetic strategy according to the literatures [25, 34-35]. Firstly, the multi-walled CNTs (MWNTs) (China NTP Corporation) or BP2000 (American Cabot materials was mildly oxidized by the acid solution of 6.0 M

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Corporation) carbon

HNO3 in a round-bottom flask, refluxing for 6 h at 100 ℃. After purification and

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desiccation, the mildly oxidized MWNTs or BP2000 carbon (about 50 mg) was dispersed in the mixture of 80 mL ethanol and 2 mL high purity water. Then, 3.0 mL

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0.5 M Co(OAc)2 was gradually added into the suspension solution and kept stirring for 12 h until the ethanol evaporated. Finally, the as prepared precursor was annealed

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at 450 ℃ for 2 h in Ar atmosphere to obtain the CoO/carbon composite. The composition of the as-prepared CoO/CNF, CoO/MWNTs and CoO/BP2000

composites were determined by ICP-MS (Agilent 7500a), and the metal cobalt content are 15.24% for CoO/CNF, 13.48% for CoO/MWNTs and 18.56% for CoO/BP2000 composite, respectively. The structure of the prepared samples was investigated by X-ray powder diffraction (XRD, Bruker D8). The morphology of the prepared sample was observed by the scanning electron microscopy (SEM, JSM-6701F) and transmission electron microscopy (TEM, JEM-2010), and the elemental analysis of the prepared sample was conducted by EDX in the TEM device. 5

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2.2 Electrochemical Tests

Electrocatalytic activities of the prepared catalyst samples were investigated using

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a Swagelok-type Li/O2 cell. The air electrodes and Li-O2 batteries were prepared as following. Firstly, a mixture slurry with a weight percentage of 45% as-synthesized

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catalyst, 5% PTFE binder, and 50% ethanol solvent was prepared and then sprayed

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onto a 20 mm circular Ni-mesh current collector, finally dried in a vacuum oven at 80 ℃for 12 h. The loading of catalyst in the air electrodes was about 2.5 mg cm-2 in our

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experiments. A Li-O2 battery was assembled in an argon filled glove box using a Li

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metal sheet as anode, double Celgard 2400 as separator, the prepared air electrode as cathode and 1.0 M LiTFSI/TEGDME solution as electrolyte. The Li-O2 battery with

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an oxygen window about 20 mm in diameter was tested in 1.0 atm oxygen

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atmosphere at 25 ℃, and the discharge and charge curves were recorded by LAND

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CT2001 battery test system. The specific capacities of Li-O2 battery were calculated based on the mass of catalyst on cathode. The electrocatalytic properties of the sample catalysts were also characterized by

rotating disk electrode (RDE) voltammetry. For RDE test, the working electrode preparation process was simply described as following: Firstly, a catalyst ink with a concentration of 5.0 mg mL-1 was prepared, and then deposited onto a glassy carbon

electrode (RRDE-3A 011169), 5.0 μL at a time for ten times. After the catalyst thin film dried, the three electrode cell used for RDE measurements was assembled in an argon filled glove box using Pt wire as the reference electrode and Li metal sheet as counter electrode. The lithium counter electrode was prepared by embedding Li foil 6

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into a nickel foam with an attached nickel wire. To prevent convective oxygen transport to Li metal, the Li counter electrode was wrapped in a celgard 2400 separator [36, 37]. The electrolyte used in this three-electrode cell was 1.0 M

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LiTFSI/TEGDME electrolyte. The RDE tests were carried on a RDE device

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(RRDE-3A, Japan) and the sweep curves for the CoO/CNF, CoO/MWNTs and

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CoO/BP2000 cathode catalysts were obtained in O2-saturated 1.0 M LiTFSI /TEGDME electrolyte at a rotate speed of 1600 rpm. The RDE result were corrected

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by the background current in this work and the background current was obtained in

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Ar-saturated 1.0 M LiTFSI/TEGDME electrolyte.

3. Results and discussion

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Figure 2a presents the XRD patterns of CoO/CNF composite and pure carbon fibers

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(CNF). As seen, the XRD pattern of CoO/CNF composite showed the diffraction peaks at

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2θ = 36.495o, 42.401o, 61.515o, 73.686o, 77.562o, which can be ascribed to the characteristic peaks of the cubic CoO phase according to PDF file #75-0533. The broad peaks appeared near 25° in the XRD patterns of both CoO/CNF and pure carbon fibers correspond to the diffraction peaks of graphite (PDF file #74-2329), indicating that the PAN fibers were converted into disordered carbon during catalyst preparation [6, 38]. The XRD patterns of the CoO/MWNTs and CoO/BP2000 composites shown in Figure 2b and 2c exhibit the same CoO phase (PDF file#75-0533) as that displayed in CoO/CNF composite. The morphologies and microstructures of the CoO/CNF composite were examined 7

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by Field-emission scanning electron microscopy (FESEM) and the transmission electron microscopy (TEM). Figure 3a & 3b show the typical SEM images of the electrospun

polymer

fibers

under

different

magnifications.

As

seen,

the

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Co(OAc)2/PAN fibers have a smooth surface with a diameter about 300 nm. In the

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process of Co(OAc)2/PAN fibers converted into CoO/CNF fibers, it was reported that

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the pre-oxidation process was necessary and could help to preserve the morphology of the nanofibers in the final carbonization treatment [6, 39]. Therefore, a pre-oxidation

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process of thermal treatment for Co(OAc)2/PAN fibers was conducted at 230 ℃ in the air atmosphere. After pre-oxidation, it can be seen that the diameter of the fibers

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becomes a little smaller than that of the pristine fibers as shown in Figure 3c & 3d. Figure 3e & 3f show the SEM images of the nanofibers after the carbonization

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treatment. It is obviously that the fibrous structure was preserved with a diameter about 200 nm. Moreover, some nanoparticles appear on these carbon fibers, which

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make the outer surface of carbon fibers become rough. Furthermore, as shown in Figure 3g, 3h & 3i, the TEM images reveal the microstructure of the carbon fibers more clearly. It was found that the size of the nanoparticles on the outer surface of carbon fibers have an average diameter about 60-70 nm. Many small pores were also generated in the skeleton of carbon fibers, which made the carbon fibers become porous. The formation of the porous structure can be attributed to the outward diffusion of resulted gases, as well as the localized weight loss of carbon, which catalyzed by the cobalt oxide during the carbonization process for the Co(OAc)2/PAN fibers [6-7, 40]. Figure 3j shows the EDX analysis of a small particle in Figure 3h, 8

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and Figure 3i shows 0.2460 nm crystal lattice distance between two adjacent fringes along the [111] direction, which is consistent with the crystal lattice characteristic of CoO reported in the previous work [41]. These results demonstrated that the cobalt

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oxide (CoO) particles were successfully loaded on the carbon fibers in the CoO/CNF

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composite. Herein, it is worth mentioning that the as-prepared CoO/CNF composite

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with a porous structure can allows the gas to go through the carbon fibers and directly contact the cobalt oxide (CoO) particles. Therefore, the utilization ratio of CoO

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nanoparticles in the carbon fibers can be greatly improved. As a result, more catalytic active sites can be obtained for the reduction reaction of oxygen (ORR) in the Li-O2

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battery.

In order to study the electrochemical performance of the above-mentioned

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catalysts in the Li-O2 batteries, Figure 4 compares the rate performance of the batteries using the CoO/CNF, CoO/MWNTs and CoO/BP2000 cathode catalysts in the

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first discharge-charge process. As seen in Figure 4d that the Li-O2 battery with the CoO/CNF cathode catalyst delivered higher capacity than those of the CoO/MWNTs and CoO/BP2000 cathode catalysts at all the investigated current densities of 0.2, 0.4, 0.8 and 1.0 mA cm-2, respectively. Moreover, the discharge profiles of the CoO/CNF

cathode show much higher voltage platforms than those of CoO/MWNTs or CoO/BP2000 cathode at all investigated current densities as shown in the inserted illustrations in Figure 4d. The round-trip efficiencies for the Li-O2 batteries with CoO/CNF cathode were also higher than those of CoO/MWNTs or CoO/BP2000 cathode. These results indicate that the CoO/CNF composite was a superior 9

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electrocatalyst for Li-O2 battery. The cycling life, as another important parameter for evaluating the electrochemical performance of catalysts in the Li-O2 batteries, was also investigated as shown in

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Figure 5. Figure 5a depicts the cycling stability of the Li-O2 battery with the

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CoO/CNF catalyst at a current density of 0.2 mA cm-2. As seen, the discharge capacity

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of the Li-O2 battery with CoO/CNF cathode still preserved about 3302.8 mAh gcat-1 after 8 cycles, and the capacity retention was about 85.1%, while only 2120.5 mAh

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gcat-1 discharge capacity (59.7%) was left for CoO/MWNTs cathode and 1605.8 mAh gcat-1 discharge capacity (55.7%) for CoO/BP2000 cathode as shown in the inserted

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illustrations in Figure 5a. These results indicate that the CoO/CNF catalyst showed superior cycling stability than those of CoO/MWCNTs or CoO/BP2000 catalyst.

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More interesting, when a constant capacity regime was employed in the cycling tests,

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the Li-O2 battery with CoO/CNF cathode cycled at a fixed capacity of 1000 mAh gcat-1

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with a current of 80 mA gcat-1 (about 0.2 mA cm-2) exhibited a stable cycling

performance over 50 cycles as shown in Figure 5b. Only 25 cycles were obtained by cycling the Li-O2 batteries with CoO/CNF cathode catalyst in the voltage range of

4.2-2.0 V as shown in Figure 5c. Poor cycling stability remains a significant challenge for the organic electrolyte Li-O2 batteries. The failure to decompose all of the reduction products on charge, lithium anode degradation and electrolyte evaporation were the main reasons limiting the cycling performance. All of the electrochemical test results mentioned above have proved that the CoO/CNF cathode catalyst showed higher electrocatalytic activity for the oxygen 10

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reduction (ORR) than those of the CoO/MWNTs and the CoO/BP2000 cathode catalysts. The excellent electrocatalytic activity of the CoO/CNF cathode catalyst mainly arises from the unique structure of the carbon support. RDE test was also

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performed to clarify this problem. The RDE tests were carried out in an O2-saturated

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1.0 M LiTFSI/TEGDME electrolyte at a rotate speed of 1600 rpm. As shown in

Figure 6, we observed that all the three electrodes presented a same onset potential of

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2.91 V (vs. Li/Li+) for oxygen reduction. This result indicated that oxygen reduction

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on CoO based electrodes proceeded under the similar kinetics mechanism which is independent on the carbon supports. While at the diffusion-controlled region (< 2.4 V

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vs. Li/Li+), we can see that the CoO/CNF delivered highest current density. This probably due to the novel structure of CNF provided more active sites for oxygen

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reaction than the other two electrodes.

In order to further illustrate the contribution of the unique microstructure of

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carbon nanofibers. Fig.7 compares the SEM images of CoO/CNF, CoO/MWNTs and CoO/BP2000 samples. It is clear that the nanofibers of CoO/CNF show obvious lager diameter and also much longer in length than the carbon nanotubes. Considering the superior electrochemical performance of CoO/CNF, it may be supposed that the carbon nanofibers may form a network with larger gas transport channels than the carbon nanotubes. In addition, table 1 shows the BET surface areas and total pore volume data of all three samples. As seen, the BP2000 carbon support presents highest surface area of 516.58 m2 g-1 due to its very small nanosize particles as shown

in Fig. 7C, and with total pore volume data of 0.8574 cm3 g-1. However, the 11

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electrocatalytic performance of CoO/BP2000 was inferior to those of CoO/CNF and CoO/MWNTs. That is because the carbon nanotubes and carbon nanofibers can form porous network with random interconnected pore structure, which can facilitate the

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gas transport for the oxygen electrode and thus increase the effective reaction area on

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the oxygen electrode. Furthermore, compared with the carbon nanotubes (117.64 m2

g-1, 0.3482 cm3 g-1), the carbon naofibers show larger surface area and higher total

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pore volume data (124.17 m2 g-1, 0.9372 cm3 g-1), which can be attributed to the

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porous surface structure of the nanofibers. As a result, the discharge capacity and cycling stability of Li-O2 battery with CoO/CNF catalyst can be greatly improved. In

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short, the unique structure of the CoO/CNF composite has greatly improved its electrocatalytic activity, resulting from the excellent gas diffusion characteristics and

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4. Conclusion

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high utilization of cobalt oxide (CoO) particles.

In summary, nanofibrous CoO/CNF composite with a diameter about 200-300 nm

was synthesized via one-step electrospinning technique and subsequent carbonization

process. The carbon nanofibers can form porous network with random interconnected pore structure, which facilitates the gas transport for the oxygen electrode. The porous surface of the nanofibers offers large area of “electrochemical reaction interface”, which also could effectively improve the utilization ratio of the CoO nanoparticles. Accordingly, the discharge capacity and cycling stability of the Li-O2 battery with CoO/CNF catalyst can be greatly improved. The results of charge-discharge tests and 12

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RDE tests demonstrated that the CoO/CNF cathode catalyst had higher electrocatalytic activity for the ORR than those of CoO/MWNTs and CoO/BP2000 cathode catalysts. The CoO/CNF composite can be used as a promising electrocatalyst

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for the Li-O2 batteries.

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Acknowledgments

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The authors are grateful for the financial support by the 973 Program of China (2014CB932303) and the Natural Science Foundation of China (21336003,

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21073120).

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application as a bi-functional cathode for rechargeable lithium-oxygen batteries,

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Chemical Society, 134 (2012) 15849-15857. [35] J.C. Kim, I.S. Hwang, S.D. Seo, D.W. Kim, Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO

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Table captions: Table 1 BET Surface Area and Pore Volume of CoO/BP2000, CoO/MWNTs and CoO/CNF

Surface Area

Pore volume

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BET Data

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cathode catalysts by BET ASAP2020 Analyzer.

516.58

CoO/MWNTs

117.64

CoO/CNF

124.17

0.8574

0.3482

0.9372

M

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CoO/BP2000

(cm3 g-1)

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(m2 g-1)

Catalysts

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Figure captions:

Figure 1 Scheme of the preparation of CoO/CNF composite, a) the electrospinning

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device applied in this work, b) Co(OAc)/PAN electrospun fibers, c) fibers after pre-oxidation at 230 ℃ for 10h in air atmosphere, and d) CoO/CNF composite after carbonization at 900 ℃  

Figure 2 X-ray diffraction patterns of a) CoO/CNF composite, b) CoO/MWNTs composite, c) CoO/BP2000 composite.

 

Figure 3 SEM micrographs of composite fibers via electrospinning, a) & b): Co(OAc)2/PAN electrospun fibers at different magnifications, c) & d): fibers after pre-oxidation at 230 ℃ for 10 h in air atmosphere; e) & f): CoO/CNF composite after carbonization at 900 ℃ for 1h in argon atmosphere. High-magnification TEM images: g) and h): CoO/CNF fibers at different magnifications, i) crystal lattice 20

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analysis of cobalt oxide (CoO). j) elemental analysis of CoO/CNF

Figure 4 Rate performance of the Li-O2 batteries with a) CoO/CNF, b) CoO/MWNTs and c) CoO/BP2000 composite catalysts at various current densities in the first

ip t

discharge-charge process, respectively; d) Comparison for the discharge capacity of these three kinds of catalysts at various current densities, and the discharge voltage

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platform, round-trip efficiencies are shown in the inserted illustrations. The electrochemical tests were carried on in a Swagelok-type cell using 1.0 M

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LiTFSI/TEGDME electrolyte in 1.0 atm oxygen atmosphere at 25 ℃ . The

an

discharge-charge voltage range was between 2.0-4.2 V.

Figure 5 a) Cycling performance of the Li-O2 batteries with CoO/CNF at a current

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density of 0.2 mA cm-2, in 2.0-4.2 V, and a comparison of cycling stability for CoO/CNF, CoO/MNTs and CoO/BP2000 as shown in the inserted illustrations. b) Cycling stability of Li-O2 batteries with CoO/CNF catalyst at a fixed capacity of 1000

d

mAh gcat-1. c) Cycling stability of Li-O2 batteries with CoO/CNF catalyst at a current

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density of 0.2 mA cm-2, in 2.0-4.2 V.

Figure 6 RDE testing results for ORR in O2-saturated 1.0 M LiTFSI/TEGDME electrolyte at a voltage sweep rate of 10 mV s-1 and 1600 rpm.

Figure 7 SEM images of CoO/CNF, CoO/MWNTs and CoO/BP2000 samples. 

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